Hop products have been used for about 2000 years to embitter beer. .alpha.-bitter acids, the so-called humulones, are constituents of hop resin and act as the main mediators of the bitter hop component. These cyclohexadienone derivatives, which are present in special cell compartments of the fruit bodies of hop umbels, are natural antibiotics and serve the hop plant as protection against being eaten.
The hop processing industry has in recent times utilized processes for the selective isolation and concentration of these special hop constituents not least for reasons of rationalization for the breweries. Developments in the field of hop processing such as drying and pellet pressing have above all provided the basis for the production of hop extracts by means of supercritical carbon dioxide.
In this process the less mild processing methods of hop powder production prior to the subsequent extraction procedure inter alia cause destruction of the natural compartmentation of various cell constituents. In hops this results above all to a substantial decrease in the humulone content that is partially caused by enzymes and can be up to 40% even under inert gas cover. In addition to the known losses of humulones by atmospheric oxygen-dependent oxidative processes, it is especially the aforementioned losses caused by processing under protective gas and the concomitant shortening of storage periods that still cause problems.
Thus German Offenlegunschrift 28 08 981 teaches a process for the production of hop pellets with a lowered water content in which a stream of air having 2 to 7% humidity and a maximum of 50.degree. C. is passed over the pellets in a second drying process. The object of the process is to stabilize the specific hop constituents by lowering the water content of the pellet; however, when the pellets are subsequently cooled to ambient temperature the pellets again take up atmospheric humidity which partially abolishes the previously attained stabilizing effect.
German Offenlegunschrift 26 27 534 also describes the advantage of the hop extract relative to other processing methods as a lowered water content down to 0 to 10%. However, this drying method makes use of the controversial solvent extraction method (methylene chloride) with all its known drawbacks. In the conventional process of mild hop drying with the object of an optimal yield of .alpha.-bitter acids, the hop umbels are dried at 60.degree. to 80.degree. C. circulating air temperature to an average residual moisture content of 12 to 14%. Due to the natural microbicidal action of the .alpha.-bitter acids obtained in this manner they largely evade destruction by microorganisms and accordingly should be stable when stored under conditions free of atmospheric oxygen.
It is above all the extraction of the bitter hop constituents by means of supercritical carbon dioxide which has essentially resulted in an appropriate treatment of hops for foodstuffs followed by increased extract yields. The advantages of CO.sub.2 extraction such as a multiple increase in the uptake of the substance to be extracted by the selective inert gas carbon dioxide or the simplified and residue-free separation of extracted substance and carrier gas are described in detail in German patent 14 93 190.
Nevertheless, it has been shown that the humulone contents in hop pellets and also in extracts are subject to a partially non-oxidative or indirect-oxidative temperature-dependent decrease.
Although processes have been described which are intended to protect hop products above all from humulone losses, these methods have without exception the drawback that they require the addition of auxiliary agents. According to German Offenlegunschrift 28 33 589 .alpha.-acids are protected against decomposition by admixing the hop extract with 1 to 3% by weight metal oxides such as calcium oxide and/or magnesium oxide whereby this hop mixture is subjected at the same time to an increase in temperature and pressure. This procedure is combined with the addition of small amounts of lower alcohols or mixtures thereof. German patent 31 39 541 teaches a process for the protection of hops by the addition of 0.4% of the antioxidative ascorbic acid.